Bonding dissimilar ceramic components

ABSTRACT

Adhesive compositions and methods for bonding materials with different thermal expansion coefficients is provided. The adhesive is formulated using a flux material, a low flux material, and a filler material, where the filler material comprises particulate from at least one of the two components being bonded together. A thickening agent can also be used as part of the adhesive composition to aid in applying the adhesive and establishing a desired bond thickness. The method of forming a high strength bond using the disclosed adhesive does not require the use of intermediary layer or the use of high cure temperatures that could damage one or both of the components being bonded together.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/585,410, filed Dec. 30, 2014, the disclosure of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This disclosure generally relates to adhesive compositions and methodsfor creating bonds to join dissimilar ceramic components together.

BACKGROUND

Ceramic materials are well known for their excellent mechanicalproperties and stability at high temperature and have been widely usedas ideal high temperature structural materials in many fields, includingin the aeronautic fields. Permanent bonding between dissimilar materialsto form hermetic seals is required in many products and components.Particularly stringent requirements are found in the manufacture ofweapon systems, and more specifically with missile flight systems.However, although such ceramic materials exhibit desirable propertiesthey may also exhibit brittleness that can restrict their application infabricating structures, especially those structures with largedimensions and complex shapes. Thus, joining of ceramic components andespecially dissimilar ceramic components can present challenges.Traditional joining or bonding technologies like mechanical connection,diffusion bonding and brazing are used for ceramic-to-ceramicconnections, however, each of these known techniques have drawbacks.

In situations where the two materials have dissimilar thermal expansioncoefficients, temperature fluctuations may induce fractures or permanentdeformation that either cause the two different materials to break apartor shift in position relative to each other. The temperature changes canreflect cooling from the processing temperature at which the parts werebonded or from temperature cycles during the lifetime of the productcontaining the bonded components. Attempts to avoid the challenges ofbonding dissimilar ceramic components have included, trying to selectmaterials such that the mismatch of the different coefficients ofthermal expansion (CTE) are minimized, performing bonding at the lowestpossible temperature to avoid residual forces that can be locked induring cooling, minimizing the bonding area, use of a compliant layerthat will absorb some of the thermal mismatch, incorporation of amulti-layer bonding system where each layer provides a gradual stepchange in the mismatched CTE, and even product design changes in anattempt to place the bond areas in locations of minimal relativemovement of the joined parts.

Although there has been limited success with known methods of bondingceramic components, challenges still exist in bonding dissimilar ceramiccomponents, especially those components with different CTEs. Forexample, the known bonding methods can place severe restrictions on thematerials that can be joined, use curing temperatures that are lowerthan thermal cycle temperatures experienced by the produced part, formlow bond strength due to minimal bond area, cause shifting of bondlayers due to thermal cycles, and involve high cost of multi-layerbonding techniques and/or design changes.

Accordingly, there is a need for improved adhesive compositions andbonding methods that avoid or minimize the issues associated with knownbonding techniques and provides a cost effective solution to bondingdissimilar ceramic components.

SUMMARY

This disclosure provides adhesive compositions and methods for bondingceramic components having different coefficients of thermal expansionusing the adhesive compositions disclosed below. Bonding according tothe present disclosure will allow ceramic components having differingCTEs to form a long lasting bond between the materials while overcomingthe aforementioned problems of the current methods. The adhesives andbonding methods disclosed will also allow for the formation of hermeticseals between ceramic components having different CTEs.

One disclosed adhesive composition for use in bonding together a firstcomponent comprising a ceramic having a first coefficient of thermalexpansion with a second ceramic component having a second coefficient ofthermal expansion that is different than the first coefficient ofthermal expansion, contains a particulate having the same chemicalcomposition as the first ceramic component, a fluxing agent, and a lowflux material. The adhesive is characterized in that application of heatto a layer of the adhesive that is in contact with bonding surfaces ofthe first and second components causes chemical composition changes tothe bonding surfaces resulting in a formed bond between the first andsecond components.

The adhesive can also include water and a thickening agent to aid in theapplication of adhesive to one or more bonding surfaces and to achieve adesired resultant bond thickness between the first and secondcomponents.

A method of joining ceramic materials having differing coefficients ofthermal expansion includes the steps of providing a first componenthaving a first bonding surface and a second component comprising aceramic having a second bonding surface, applying an adhesive to one orboth of the first and second bonding surfaces to form a bond layer,where the adhesive comprises particulate having the same composition asone of the first or second components, a fluxing agent, and a low fluxmaterial. Once the adhesive is applied, the first and second bondingsurfaces are joined with the bond layer and pressure is applied to oneor both of the first and second components in a direction perpendicularto the adhesive layer to form an adhered composite. An initial heat ofabout 300° F. maximum temperature is applied on the joined adheredcomposite to drive off any moisture and/or other volatile componentsfrom the adhesive mixture, while establishing sufficient green strengthto keep the joined components adhered to one another. A final heatingstep is used to cause chemical compositional changes to the adhesive andat least one of the substrates. Final heating of the adhered compositeis accomplished through a step temperature profile starting at about450° F. for at least 30 minutes, and finishing at a minimum of about2,000° F. for 30 minutes.

The method can also include an adhesive formulation step where water, achemically modified cellulose thickening agent, lithium metaborate, andzirconium oxide, and particulate having the same composition as thefirst component are mixed to form a homogenous admixture.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments, the further details of which can be seen withreference to the following description and drawings.

BRIEF DESCRIPTION OF THE FIGURES

The present disclosure will become more fully understood from the moredetailed description presented below and the accompanying drawings whichare presented by way of illustration only, and thus, are not limitationsof the present disclosure, and wherein:

FIG. 1 is a schematic illustration of the adhesive of this disclosureapplied as a layer on a bonding surface of a second component prior tocontact with the bonding surface of a first component;

FIG. 2 is a schematic illustration of the bonded components of FIG. 1showing that the bonding surfaces of at least one component has changedin chemical composition; and

FIG. 3 is a graphical representation of data obtained from ESCA analysisof a bonded substrate.

DETAILED DESCRIPTION

Joining two components of ceramic materials having differentcoefficients of thermal expansion (CTE) starts with knowing the chemicalcomposition of each component. FIG. 1 schematically illustrates that afirst ceramic component 10 is to be joined to a second ceramic component14. The ceramic components can include traditional ceramic materials,defined as inorganic, nonmetallic solids prepared by the action of heatand subsequent cooling. These ceramic materials may have a crystallineor partly crystalline structure, or may be amorphous (e.g., a glass). Asused herein the term ceramics also can include ceramic matrix composites(CMC), which are a subgroup of composite materials as well as a subgroupof technical ceramics. They consist of ceramic fibers embedded in aceramic matrix, thus forming a ceramic fiber reinforced ceramic (CFRC)material. The matrix and fibers can consist of any ceramic material,whereby carbon and carbon fibers can also be considered a ceramicmaterial. Also included in the term ceramics are metal matrix ceramics(MMC), sometimes referred to as “metaloceramics” that typically compriseabout 60% metal alloy (aluminum, magnesium and/or titanium) and 40%traditional ceramics (typically Al₂O₃). These MMCs are characterized inthat they are non-porous, machinable and can withstand high temperaturesthat typical traditional ceramics cannot.

The two ceramic components 10 and 14 illustrated in FIG. 1 could includerespectively, a MMC, such as an alumina containing silicon carbidewhiskers and a CMC, such as, a glass coated CMC. For example, the glasscoating ceramic component 14 could comprise a specialty glass containingSiO₂, CaO and Al₂O₃. One such glass is VIOX 2936 manufactured and soldby Ceradyne, Inc. Each of the ceramic components 10 and 14 also have abonding surface 12 and 16. These bonding surfaces are typically bondedas received with no surface treatment. One advantage of the adhesive andmethod of bonding disclosed here is that an intermediate layercomprising a solid sheet of material sandwiched between the two ceramiccomposites is not needed. Only a homogenous adhesive is required.

The adhesive layer 18 can be applied to either bonding surface 12 or 16,or both. First, however, the adhesive is prepared by starting withparticulate that has the same chemical composition of one of the ceramiccomponents 12 or 14. One way to obtain the particulate is to take aportion of one of the ceramic components and to grind the sample portionto a desired particle size. Grinding to a particle size of from about200 mesh to about 325 mesh will allow uniform distribution with theother constituents of the adhesive. Grinding can be accomplished in anumber of ways, for example, when only small amounts of adhesives areneeded then a standard mortar and pestle can be used. For larger amountsof needed adhesive, automated mechanical grinders can be used. Groundparticles from each ceramic component can also be used to prepare theparticulate constituent of the adhesive. The total amount of particulateused in the final adhesive formulation is preferably determined toapproximate or match the density and CTE of the base material.Typically, the particulate is in the range of from about 20 wt. % toabout 75 wt. % of the adhesive. The adhesive also contains a fluxingagent and a low flux material.

The fluxing agent includes one or more of the following known fluxingagents, lithium metaborate, sodium carbonate, boron carbide, high borateglass, lime, alkaline ash and/or a mixture of these agents. Thesefluxing agents lower the melting point or softening temperature of thechemical matrix located at the bonding surfaces of the ceramiccomponents to be bonded. The fluxing agents interact with the surfacemolecular structure of the bonding surfaces and pull them away (dissolvethem) molecule-by-molecule. The fluxing agent can be ground to about thesame size as the particulate. The total amount of fluxing agent used inthe final adhesive formulation is in the range of from about 25 wt. % toabout 80 wt. %.

The low flux material is selected such that it does not exhibit fluxbehavior at the fired temperature used to form the bond betweensubstrates when using a specific adhesive formulation. In some cases amaterial that exhibits non-flux properties, used in the adhesivecomposition can be selected from the group comprising zirconia oxide,silicon nitride, silicon carbide, aluminates, silicates,alumino-silicates, titanates, zirconates, and any mixture of thesematerials to obtain the proper melting temperature and/or CTE gradient.Other nitride and carbide containing compounds can be used provided theyexhibit non-flux properties can be used in addition to those listedabove. The low flux material provides a structural network for the bondultimately formed from the adhesive. Additionally, the presence of thelow flux material provides strength, a CTE gradient between the bondedmaterials, thermal stability and porosity. Again, the particle size ofthe low flux material should be generally about the same size as theparticulate and fluxing agent. The total amount of low flux materialused in the final adhesive formulation is in the range of from about 20wt. % to about 75 wt. %.

Also advantageous for use in preparing the adhesive is to include achemically modified cellulose, known as cellulose ethers. Water as wellcan be used to wet the modified cellulose. These modified cellulosematerials function as stabilizers, thickeners and viscosity agents andare comprised of organic polymers comprising polysaccharides thateventually volatize during the bond curing heat treatment. The modifiedcellulose thickens the adhesive mixture by increasing the viscosity,thus allowing a desired thickness of the adhesive layer 20 to beachieved, and ultimately, determining the final bond thickness of thefinished bonded composite. The modified cellulose also provides alimited amount of bond strength of the adhesive to adhere the componentstogether before heat treatment to form the final bond 26. One particularuseful modified cellulose is methyl cellulose or Methocel™ manufacturedand sold by Dow Chemical. However, other organic thickening agents couldbe used provided they readily burn out at the calcination temperaturesused, for example, ethyl cellulose, cellulose, flours, grains or wood.These materials could be either synthetic or natural or mixtures ofboth. The total amount of modified cellulose used in the final adhesiveformulation is in the range of from about 0 wt. % to about 20 wt. %.Water, preferably cold di-ionized water, can be added to the adhesiveadmixture to achieve the desired viscosity necessary to apply theadhesive to one or both ceramic components 10 and 14. The desiredadhesive layer thickness is dependent on a number of variables with onecriteria being that it is desired to have a very thin bond width orbondline on the resultant final adhered composite after the second heattreatment. Preferably the amount of adhesive applied to one or bothcomponents will result in a layer of adhesive having a thickness of lessthan 1 mil, or 0.001 inches, especially if the materials to be bondedhave similar CTE properties. When there is a greater difference in theCTE properties, the layer of adhesive will generally be thicker.

Once the adhesive admixture is prepared an adhesive layer 18 can beapplied to one or both of the bonding surfaces 12 and 16 to a desiredthickness 20 in accordance with the criteria mentioned above. Thethickness of the adhesive layer is a function of physical sizenecessity, porosity of the materials being joined, and the CTEdifference between the two materials. The two bonding surfaces 12 and 16of the ceramic components 10 and 14, respectively, are joined, or mated,together and a pressure force is applied perpendicular to the adhesivelayer. For example in FIG. 1 this direction is illustrated bydirectional arrow 50. Of course, the pressure force could be directed inthe opposite direction against ceramic component 14 or the pressurecould be applied against both ceramic components. The amount of pressureand the time the pressure is applied is sufficient to adhere the firstand second components together for handling purposes, particularly fortransport to and placement in a curing apparatus such as an oven, kiln,calciner, autoclave or similar heating apparatus. Pressure is preferablyapplied with enough force to maintain a minimum adhesive layer thicknesswhile minimizing the possibility of the two joined ceramic components 10and 14 shifting relative to each other.

FIG. 2 schematically illustrates the formed bonded composite of ceramiccomponents 10 and 14 having a formed bond 26. This formed composite isobtained after the adhered first and second components 10, 14 asillustrated in FIG. 1 are first heat treated to a temperature of fromabout ambient room temperature to about 800° F. for about 30 minutes andthen the furnace is allowed to cool to room temperature at its own rate.During this first heat treatment or drying phase, solvents are removedfrom the adhesive formulation and the required green strength isestablished between component 10 and component 12. The term “greenstrength” as used herein describes the bond strength before the joinedand adhered composite is subjected to the final heating step at atemperature starting at about 450° F. for at least 30 minutes, andfinishing at a minimum of about 2,000° F. for 30 minutes. This finalheating step results in formation of final bond strength. The first orinitial heating of the adhered composite causes moisture and/or othervolatile components (solvents, organic materials, etc.) to be driven offfrom the adhesive layer between the substrates to establish greenstrength of the bond layer. Once green strength is achieved, the adheredcomposite can be handled without distortion, damage, separation,delamination, alteration, or other physical changes to the adheredcomposite.

The adhered composite is then subjected to a second heat treatment. Thissecond heat treat, referred to as firing or curing of the adheredcomposite composite is accomplished through a step temperature profilestarting at about 450° F. for at least 30 minutes, and finishing at aminimum of about 2,000° F. for 30 minutes. This curing step causes thefluxing agent to lower the melting point of one or both of the materialsthat are joined, without either of the materials completely losing theirstructural form. As the adhesive layer 18 is cured to form bond 26, theadhesive can transform one or both of the bonding surfaces 12 and 16into surfaces 22 and 24. These surfaces are transformed such that thechemical make-up of one or more of the bonded/joined materials exhibitsa transitional chemical gradient between the bonded surfaces. Forexample, when a SiC—Al2O3 material is bonded to a glass CMC surface, thehigh concentration of SiC body transitions into a lower C content as itapproaches the bond area due to the high temperature of thecuring/calcination step, the release of free carbon and the increase increation of SiO2. A gradual increase in the concentration of the fluxagents, such as Li and B, are also observed. Using Electron SpectroscopyChemical Analysis (ESCA) these transitional chemical gradients can bemeasured thus evidencing that one or more of the bonding surfaces havechemically changed when compared to the chemical make-up of the virginsubstrate materials being bonded together.

Example

Laboratory sized samples of two different substrate materials werebonded together using the following procedure. Lithium metaborate,zirconium oxide and frit were ground using a bench mortar and pestle tofurther reduce particle size of the powdered constituents to a uniformsize and to achieve a homogenous mixture. Small amounts of de-ionizedwater were added drop-wise into the ground powders for wetting purposesand to hydrolyze the surface. Methocel™ was added to the wetted powderformulation as thickening agent to tailor viscosity of the resultantadhesive mixture to achieve a desired bondline. The adhesive had thefollowing composition: lithium metaborate—55.6 wt. %; zirconiumoxide—5.6 wt. %; frit (Viox 2936)—38.9 wt. %; di-ionized water plusMethocel™ to achieve desired viscosity.

A thin layer of the adhesive mixture was spread across the surface of afirst substrate comprising alumina with silicon carbide whiskers(designated Substrate 1). A second substrate comprising a plate of glasscoated Ceramic Matrix Component (CMC) (designated Substrate 2) wasplaced on the adhesive layer and pressure was applied to distribute theadhesive uniformly between the two bonding surfaces and to ensure fullcontact of both surfaces of the two substrates with the adhesive layer.The joined substrates were then subjected to a two step heat treatmentas follows: A first heating in a furnace to a temperature of from aboutambient room temperature to about 800° F. and then a hold at the maximumtemperature for about 30 minutes. Allowing the furnace to cool to roomtemperature at its own rate. The second heat treatment was accomplishedthrough a step temperature profile starting at about 450° F. for atleast 30 minutes, and finishing at a minimum of about 2,000° F. for 30minutes. After cooling it was observed that the two substrates weresecurely bonded together.

Analysis of resultant bonded substrate was conducted using ESCA todetermine if the chemistry of first substrate, Substrate 1, had changedas a result of the bonding procedure. The ESCA analysis is shown in FIG.3. The sample profile locations are graphically illustrate in FIG. 2.The above ESCA analytical results demonstrates the change in chemistryfrom the virgin area of the first substrate to the bonded surfaceindicating that the substrate underwent a chemical change in the densematerial that is usually very hard to bond.

The foregoing description of the specific embodiments will reveal thegeneral nature of the disclosure so others can, by applying currentknowledge, readily modify and/or adapt for various applications suchspecific embodiments without departing from the generic concept, andtherefore such adaptations and modifications are intended to becomprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology herein is for the purpose of description and not oflimitation.

What is claimed:
 1. A composite object comprising: a first componentcomprising a ceramic having a first coefficient of thermal expansionwith a second component comprising a ceramic having a second coefficientof thermal expansion that is different than the first coefficient ofthermal expansion; a formed bond adhering the first and secondcomponents together, the formed bond resulting from heat treating anadhesive comprising: a) a particulate having the same chemicalcomposition as the first component; b) a fluxing agent; and c) a lowflux material, the adhesive characterized in that heating of theadhesive causes a chemical composition change to at least one bondingsurface of either the first or second components.
 2. The composite ofclaim 1 where the formed bond between the first and second componentsestablishes a coefficient of thermal expansion transition region betweenthe first and second components.
 3. The composite object of claim 1where the percent by weight of the particulate of the adhesive isdetermined by the difference between the coefficient of thermalexpansion of the first component and the coefficient of thermalexpansion of the second component.
 4. The composite object of claim 1where the fluxing agent comprises lithium metaborate.
 5. The compositeobject of claim 1 where the low flux material comprises zirconium oxide.6. The composite object of claim 1 wherein the adhesive furthercomprises water and a thickening agent.
 7. The composite object of claim6 where the thickening agent further comprises chemically modifiedcellulose.
 8. The composite object of claim 1 where the particulatefurther comprises particles having the same chemical composition as thesecond component.
 9. The composite object of claim 1 where the formedbond between the first and second components establishes a coefficientof thermal expansion transition region between the first and secondcomponents.
 10. The composite object of claim 1 where the percent byweight of the particulate of the adhesive is determined by thedifference between the coefficient of thermal expansion of the firstcomponent and the coefficient of thermal expansion of the secondcomponent.
 11. A method of joining ceramic materials comprising thefollowing steps: (a) providing a first component comprising a ceramiccharacterized by a first coefficient of thermal expansion and having afirst bonding surface; (b) providing a second component comprising aceramic characterized by a second coefficient of thermal expansion andhaving a second bonding surface, where the second coefficient of thermalexpansion is different than the first coefficient of thermal expansion;(c) applying an adhesive to one or both of the first and second bondingsurfaces to form a bond layer, where the adhesive comprises, i)particulate having the same composition as one of the first or secondcomponents; ii) a fluxing agent; and iii) a low flux material; (d)joining the first and second bonding surfaces with the bond layer; (e)applying pressure to one or both of the first and second components indirection perpendicular to the adhesive layer to form an adheredcomposite; (f) heating the adhered composite to form a bond between thefirst and second components, where the adhesive layer during heatingcauses chemical composition changes to at least one the first and secondbonding surfaces.
 12. The method of claim 11 where the adhered compositeis heated to a temperature of at least about 2,000° F. and held at thetemperature for at least about 30 minutes.
 13. The method of claim 11where the adhesive is formulated by mixing together water, a chemicallymodified cellulose thickening agent, lithium metaborate, and zirconiumoxide, and the particulate having the same composition as the firstcomponent.
 14. The method of claim 11 where the fluxing agent compriseslithium metaborate.
 15. The method of claim 11 where the low fluxmaterial comprises zirconium oxide.
 16. The method of claim 11 whereinthe adhesive further comprises water and a thickening agent.
 17. Themethod of claim 16 where the thickening agent further compriseschemically modified cellulose.
 18. The method of claim 11 where theparticulate further comprises particles having the same chemicalcomposition as the second component.
 19. The method of claim 11 wherethe formed bond between the first and second components establishes acoefficient of thermal expansion transition region between the first andsecond components.
 20. The method of claim 11 where the percent byweight of the particulate of the adhesive is determined by thedifference between the coefficient of thermal expansion of the firstcomponent and the coefficient of thermal expansion of the secondcomponent.